The present invention relates to a rotary polygon mirror which is used in an optical mechanism of a copying apparatus, a facsimile apparatus, etc.
A conventional rotary polygon mirror is in the shape of a prism with a plurality of side faces 1, an upper face 2 and a lower face 3 in the predetermined thickness, as shown in FIGS. 7 and 8. A reflecting mirror face formed at the side face 1 is fixed to a rotary shaft through a mounting hole 4 penetrating the center of the upper and lower faces 2 and 3.
The flatness of the reflecting mirror face of the rotary polygon mirror is particularly important. Since the polygon mirror is rotated at high speeds when in use, i.e., 3,000-15,000 rpm, the side face 1 or a vertex 5 is outwardly expanded by centrifugal force during use. As plotted by a phantom line in FIG. 8, however, the amount of the deformation or expansion 36 generated at this time differs depending on the position of the side face 1 or the vertex 5 in the heightwise direction of the prism, leading to the distortion D of the side face 1 or the vertex 5. The amount of the distortion D is the difference between the maximum and minimum values of the amount of the deformation in a direction of a normal on a line of intersection of the effective reflecting range of the reflecting mirror face formed at the side face 1 with a plane including the rotary shaft.
It is necessary to lessen the above-described distortion D because the distortion D degrades the accuracy of the optical mechanism. For this purpose, the side face 1 can be provided with a specific curved surface to negate the distortion D in the stationary state and form a plane when deformed during the rotation.
The rotary polygon mirror can have various kinds of angles in the shape of a prism, and moreover, due to the hole 4 formed for the rotary shaft to penetrate the center of each face 2, 3, the distance from the center of rotation becomes different depending on the position of the hole 4 in the peripheral direction on the side face 1. Moreover, the distance between the outer circumference of the hole 4 and the side face 1 is different depending on the position of the hole 4 in the peripheral direction on the side face 1. Therefore, the scale of centrifugal force acting on the side face 1 and the degree of the expansion of the side face 1 are varied depending on the position on the side face 1 in the peripheral direction about the hole 4, causing the amount of the deformation of the side face 1 to be varied in a complex manner at various parts of the side face 1. As such, in the above-described method to negate the distortion, the surface of a mold for forming the side face 1 is required to be processed into a complicated surface, and the process is difficult, with much time and cost consumed.